The energy requirement for the heat treatment of metal parts made of steel and/or nonferrous metals is determined basically from two heat balance relations, provided that in the case of furnaces heated only by combustion of fuel the losses relating from the removal of the combustion products are disregarded: EQU Energy requirement E=useful heat flow Q.sub.U +wall heat flow Q.sub.W.
The wall heat flow Q.sub.U (wall losses) can be lowered by suitable furnace construction and installation. The useful heat flow Q.sub.N depends upon the throughput, the heat treatment temperature and the heat capacity of the useful material that is treated. Upon entry into the cooling zone the useful material has the temperature which prevails in the zone in which the material is held at the prescribed treatment temperature and that temperature is lowered in the cooling zone to ambient temperatures by heat removal. The heat there removed is usually lost, because up to now the equipment expense for recovery of heat involved in the heat treatment of metallic articles aggregated or stacked as they are moved through a furnace appeared to be substantially too large.
It is known in practice to provide separate preheating and cooling chambers associated with the furnace proper and to carry heat by means of a gas stream from the cooling to the preheating chamber, thus in countercurrent to the advancement of the useful material through the chambers of the furnace. In the case of pass-through furnaces, however, gas-tight doors are then necessary, by which the transport of the useful material is interrupted, thus making the overall operation of the furnace more difficult. Apart from the expense, such a division into separate individual chambers is quite impossible in certain furnace types (for example, furnaces incorporating a conveyor belt). There are also furnaces for treatment of metallic material which have a combined preheating and cooling zone in which heat transfer from the material to be cooled and to the material to be preheated is intended to be obtained by convention in the furace atmosphere which is uncontrolled as to its flow path. This uncontrolled gas convection, however, permits only a very incomplete heat recovery.
Finally, it is known from U.S. Pat. No. 4,093,195, in the case of a carburizing furnace having a heating zone, a carburizing zone and a diffusion zone, to provide at least one blower disposed at the furnace side wall below the grate in each of these zones for producing a gas flow through material to be cooled directed in transverse countercurrent. Each of these blowers, however, is limited to providing the corresponding supply of air to the particular zone with which it is associated; a uniform flow of gas through the high temperature material can be produced only in the particular zone. Heat recovery is not possible in this carburizing furnace, for use in preheating.